The conversion of light energy into electrical energy through the use of photovoltaic systems has been known for a long time. These photovoltaic systems are increasingly used in the residential, economic and industrial fields. Photovoltaic systems typically include—among other components—solar panels which generate a high direct current when several are connected in a series (=photovoltaic string), as well as an inverter which converts the direct voltage into alternating voltage (e.g., single phase or triple phase current). The number of solar panels which are connected in a string is usually selected in accordance with the desired open-circuit voltage of the string at a low environmental temperature (e.g., −10° C.). It is often desirable to design and operate photovoltaic strings which put out the highest possible voltage and with this, a lower current in order to reduce costs for elements of high current tolerance as well as energy losses during the transfer of electricity. Based on existing standards and on a permissible maximum voltage of the string, the maximum open-circuit voltage is currently limited to 1000V.
Inverters are available for a large variety of voltage tolerances, wherein the costs of the inverter and its loss performance increase as the voltage tolerance rises.
The inverters, which are available on the market today, feed in between a phase and the zero conductor. The available three-phase inverters include three single-phase inverters within a housing which then feed into all three phases and against the zero conductor. The most modern inverters for solar panels are laid out for a maximum direct voltage open-circuit voltage of 850V. The load voltage of the solar panel (i.e., when they are connected to a current collector), is, however, considerably lower than the open-circuit voltage. At an open-circuit voltage of 850V, a maximum load voltage in the range of 650V is possible. Direct voltage of at least 650V in the partial load range is required to feed into a three-phased 400V network via an inverter which is laid out in three phases, taking the normal tolerances and the voltage drop at the filter chokes into account. To allow this, the system must be laid out for an open-circuit voltage of at least 1100 V; however, this is not permissible according to the standards. When complying with the standards, therefore, the amount of available voltage is insufficient to perform, e.g., a three-phase infeed into a 400 V network without an additional boost converter. At a network voltage of 480 V, a load voltage of 710 V and an open-circuit voltage of 1200 V are therefore required.
US 2009/0167097 provides an interface which is in operation solely during a start and end phase to gradually connect or disconnect the photovoltaic string to and from the inverter. The photovoltaic string is gradually placed under load; consequently, the voltage of the system is reduced from an initial voltage (open-circuit voltage) to a lower voltage (e.g., approximately an optimal voltage for the inverter). As soon as the photovoltaic string is under load and the voltage of the photovoltaic string has been reduced from its initial voltage, the interface connects the photovoltaic string to the inverter. Consequently the inverter is not exposed to the possibly damaging open-circuit voltages of the photovoltaic string. In accordance with this invention, the efficiency of the system is significantly improved as compared to the state of the art since the inverter is made from silicon, which is operated at lower voltages than the open-circuit voltage of the photovoltaic string.